Over 265 million used tires are discarded each year in the United States. Some of these tires are recycled. Most, however, end up as litter that clutters landfills. The disposal of used tires poses a serious health and environmental threat. Used tires are not biodegradable and thus are placed in landfills and stockpiles, where they provide insect breeding grounds and are potential fire hazards. The tires are also difficult to store in landfills because they tend to work their way to the top of the fill causing damage to the cap or seal of the landfill. The cost associated with discarding tires in sanctioned dump sites results in a large number of tires being dumped illegally. The problems associated with discarding used tires have been the driving force behind finding methods to recycle used tires.
Another consideration contributing to the need for the development of a reclaiming process for tires has been to recover the components used in tire manufacturing. Crumb rubber reclaimed from recycled rubber can be used as an additive to asphalt and other paving compositions, and on playing fields, in addition to being used in other manufacturing processes such as manufacturing new tires. Metal products reclaimed from recycled tires can be used in making low grade steel products such as hangers and other wire products.
These factors have led to increased demands for efficient and cost effective methods of tire recycling. A variety of techniques have been employed to process elastomers into crumb, particularly with vulcanized rubber. Heat, cryogenic, and pressurized processes are often used with some degree of success. Solvents such as acetone, gasoline and other petroleum products have been used to soften the rubber but have altered the physical properties of the elastomer. Magnets have been used in elaborate processes to extract steel from the steel belts of the used tires.
Breaking up tires into small tire chips represents the first step in the process of reclaiming materials from used tires. Schmidt, U.S. Pat. No. 5,127,588 discloses an apparatus that uses cutting blades to reduce vehicle tires into small chip-like particles.
Adkins, U.S. Pat. No. 5,905,095 discloses an approach of processing used tires by submerging shredded tires in a bath containing a combination of isocyanide, polyurethane, latex and soybean oil and heating the resultant mixture at approximately 700° F.
Lovett, Jr., U.S. Pat. No. 4,025,990 discloses a process for reclaiming rubber from whole or shredded tires by cooling scrap tires in a cryogenic freezer to the embrittlement temperature, followed by magnetic separation and further cryogenic reduction.
Another approach is shown in Hunt U.S. Pat. No. 5,362,759, which uses d-limonene, reduced pressurization and heat generated by microwave. Hunt's approach is not economically attractive on an industrial scale because the process takes over 24 hours of processing before the tire pieces are ready to grind into crumb. It also fails to even partially de-vulcanize the rubber elastomer. Hunt's approach is active and time consuming which increases the production costs of the reclaimed rubber.
Using d-limonene alone, without heat or pressurization, to soften the elastomer takes approximately 24 hours. Acetone has been used in the past in conjunction with d-limonene to speed up the process. However, acetone causes the rubber elastomers to lose its physical properties, and increases ash byproducts. All of the above-described processes are either time consuming, costly or ineffective, and therefore are of limited value.
An alternative simple process is offered that does not require heat, pressure or cryogenic methods, nor chemicals that alter the physical properties of the elastomer. This improved process passively solubilizes vulcanized rubber sufficiently to be ground into crumb in less than 4 hours. The invention uses two solvents to treat vulcanized rubber, dimethylsulfoxide (DMSO) and d-limonene. The DMSO increases the permeability of the rubber; thereby, increasing the surface area for oxidization of the elastomer by DMSO and d-limonene. As a result, processing time is reduced at least 50% at ambient temperature.
The present invention avoids the deficiencies of the prior art and provides a relatively fast, inexpensive, and effective method for recovering recyclable materials from used tires and other elastomeric materials. The present invention is directed at solubilizing the elastomer without heat, pressure, or cryogenic methods or the use of chemicals that alter the elastomer's physical properties. Other objects and advantages of the invention will appear from the following description.